Oroville Dam inspectors ignored integrity of hillside that eroded

Water flows out of the damaged spillway at Oroville Dam in Oroville, Calif., on Tuesday, February 14, 2017.

Water flows out of the damaged spillway at Oroville Dam in Oroville, Calif., on Tuesday, February 14, 2017.

Photo: Scott Strazzante, The Chronicle

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Water flows out of the damaged spillway at Oroville Dam in Oroville, Calif., on Tuesday, February 14, 2017.

Water flows out of the damaged spillway at Oroville Dam in Oroville, Calif., on Tuesday, February 14, 2017.

Photo: Scott Strazzante, The Chronicle

Image 3 of 3

Staging site for construction supplies near Oroville Dam in Oroville, Calif., on Tuesday, February 14, 2017.

Staging site for construction supplies near Oroville Dam in Oroville, Calif., on Tuesday, February 14, 2017.

Photo: Scott Strazzante, The Chronicle

Oroville Dam inspectors ignored integrity of hillside that eroded

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Inspectors visited Oroville Dam 14 times since 2008 but never considered the integrity of the hillside that eroded below the emergency spillway, leading to a near catastrophe that forced the evacuation of nearly 200,000 people downstream, state records show.

The inspections by the state’s Division of Safety of Dams repeatedly mentioned the concrete apron, or weir, at the top of the emergency spillway, and often included photographs of it.

“The structure was stable appearing, and the concrete remains sound,” inspectors wrote after their latest visit to the state-run dam in August 2016.

But over and over, there is no mention of the expanse of bare earth below, even though environmentalists had raised deep concerns about the emergency spillway not having a paved lining to carry water behind the nation’s tallest dam to the Feather River.

The lack of hillside studies could have been disastrous, given that the spillway was rated to handle water flows of hundreds of thousands of cubic feet per second, but began washing away over the weekend — and threatening to undermine the rim of the dam — when a comparative trickle washed over it for a matter of hours.

The emergency spillway at Oroville was pressed into duty after the main spillway was damaged last week amid rainstorms. It was the first time the emergency spillway was used since the dam 75 miles north of Sacramento was completed in 1968.

John Cassidy, an ex-officio member of the U.S. Society on Dams and a retired engineer at Bechtel Corp. who lives in Walnut Creek, said emergency spillways are not where dam designers typically invest limited resources.

“They usually don’t operate,” he said. “That’s the whole thing: They come into play only when you have a really large flood. The money is spent on the main spillway.”

Still, he said, alternative spill channels like the one that failed at Lake Oroville should work — regardless of how infrequently they’re used.

“Certainly the area downstream of the crest of the emergency spillway (at Oroville) should be investigated thoroughly,” Cassidy said, with operators using the information to decide whether the hillside can sustain storm damage and whether all or part of it it should be paved.

Nicholas Sitar, a UC Berkeley professor of civil and environmental engineering, said it appears officials misunderstood what a heavy flow of water would do to the “natural slope” beneath the emergency spillway.

“Unfortunately, it wasn’t surprising to me that erosion happened at much lower capacities, which has happened in other rock channels,” Sitar said. “The problem of rock erosion in spillway channels is a known phenomenon.

The California Department of Water Resources, which runs the dam, said it had not anticipated that erosion below the emergency spillway would quickly threaten the spillway itself.

“The slow-moving water found a spot close to the wall, and that’s where the erosion started,” said Chris Orrock, a spokesman for the agency. “It caused erosion that we didn’t expect. We weren’t planning on this. Since (the emergency spillway) had never been used, we didn’t know how the water was going to affect the hillside.”

More than 180,000 people were evacuated Sunday in downstream communities amid fear that scars on the hillside created by erosion would burrow through the hill and under the concrete weir, causing a wall of water to descend upon several communities.

The evacuated residents were allowed back into their homes Tuesday after the Department of Water Resources drew down the reservoir to a level well below the lip of the emergency spillway — which the agency hopes not to use again this season. At the same time, workers sought to shore up the hillside erosion with rocks brought in by trucks and helicopters.

Bill Croyle, the acting director of the agency, said 100,000 cubic feet per second of water was being sent down the damaged main spillway, which was holding together. He said he expected to bring the lake down to 850 feet above sea level by this weekend, or 50 feet below the emergency spillway.

While the emergency spillway sends water over bare ground, the hillside was supposed be able to handle a significant flow during a flood, according to the state — up to 500,000 cubic feet per second.

But over the weekend, only 12,600 cubic feet per second of water poured over the weir at the peak. Within a day, the hillside was in danger.

The magnitude and swiftness of the failure at Oroville has prompted engineers and environmental groups to question why the state didn’t do more to prevent the problem.

The inspection records dating back to 2008 indicate only cursory looks at the concrete spillway lip. Many of the inspection reports say the emergency spillway weir “is in good condition” or “remains in satisfactory condition” or remains “unchanged from recent inspections.”

“There were concerns about seepage from the reservoir itself, but nothing that went to the point that there was a need to assess surfacing of the auxiliary spillway,” said Robert Bea, a professor emeritus and engineering expert at UC Berkeley, who reviewed the latest 14 dam inspection reports.

And it’s not as if the state hadn’t been alerted. In 2005, three environmental groups — Friends of the River, the Sierra Club and the South Yuba River Citizens League — warned state and federal water regulators about the emergency spillway in a 31-page motion filed with the Federal Energy Regulatory Commission during a relicensing review.

The groups were concerned that use of the unpaved auxiliary spillway would cause erosion that could lead to “a loss of crest control,” or failure of the rim of the dam. That’s exactly what almost happened, said Ronald Stork, senior policy director for Friends of the River.

“No spillway of any kind in the United States should suffer a loss of crest control,” said Stork. “What was just astonishing to me was how that rock was just eroding away as if it was clay.”

Sitar said, “When they relicensed the structure, they probably looked at the past performance of the (main) spillway and said it’s doing well. The chances we’ll ever use the auxiliary spillway are low, and there’s no need to worry about it.”

In the end, the Federal Energy Regulatory Commission dismissed concerns about the emergency spillway.

“The emergency spillway meets FERC’s engineering guidelines for an emergency spillway,” said John Onderdonk, then a senior civil engineer, in a 2006 memorandum. “The guidelines specify that during a rare flood event, it is acceptable for the emergency spillway to sustain significant damage.”

Strengthening the hillside may have been seen as too expensive. Jay Lund, a professor of civil and environmental engineering at UC Davis, said there may be more cost-effective ways to increase the reservoir’s discharge capacity.

Auxiliary spillways like the one at Lake Oroville, discharging water into an unpaved hillside, are not the norm. But others exist.

The federally run New Melones Dam on the Stanislaus River, which encloses another of California’s biggest reservoirs, has a similar spillway, where water is designed to pour down a bedrock slope once the lake level reaches a certain point. The emergency channel has never been used.

Hetch Hetchy Reservoir in Yosemite has many spillways, one of which is unlined and sits on granite. The gated chute is used regularly and has never had a problem, according to the San Francisco Public Utilities Commission.

Dan Wade, the agency’s director of capital improvements, said there are many factors to consider when designing an unpaved spillway, including the soil’s potential for erosion, how often it will be used and how much water will run over it.

“Unlined spillways are not uncommon,” Wade said. “Judgment calls have to be made by the design engineering team.”